The death of a classic

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Finn Hammer by way of Terry Fritz <twftesla-at-qwest-dot-net>" <f-h-at-c.dk>

Perhaps it is not so good an idea to fit a museum coil with a MMC style
capacitor.

The capacitor for The Museum Coil is a classic, in that it was one of
the first really big MMC`s 3 years back. It has put up with all my
initial mistakes, a lot of abuse, and an estimated 10 hours of total run
time. I had thought it would last 470 hours, but alas, I must have been
treating it worse than I thought I did.

Recovering from a Weekend bicycle race yesterday (160 miles -at-9h22min)the
museum manager called me this morning to tell me, that the coil was
smelling.
"Red Alert"
So I hurried down to the museum, carrying with me the cap from the
Ambassador coil in order to have something to rig up, so the visitors
could get what they came for.
Now it is gone for good.

http://www.wimshurst-dot-com/mmc1.jpg

http://www.wimshurst-dot-com/mmc2.jpg

What went wrong?
Perhaps I just pounded it too heavily, I didn`t always hook up the
fiberprobes, so I really have no idea about how often they were
overvolted.
They never were hot, even one time when they ran for an hour, without
stop, during EMC measurements.
So probably, one cap gave up, and this lead to heat inside it, being
transmitted to nearby caps, and the avalance started.

This is the reason that I will not use a MMC again on a professional
coil: It is not reasonable to ask, that the operator should be able to
spot that first bulging cap when it pops out amongst the others, A pro
coil damands a pro cap.

With the caps from the Ambassador, rearranged for 28nF, and 5 strings
from the original MMC, i managed to get the coil up in an hout or so,
and am now anxiously awaiting the arrival of the backups, Maxwell 37667,
that I bought from Tesla Systems Research.
Got 6 of those, and am going to use 4 in a quad, and 2 as spare. Then it
is only my hope that they will last untill we get the Maxwell 35309,
which is a 40nF 100kV cap.

Don`t ever deal with a professional setup without a backup! 

Cheers, Finn Hammer